Oil-water interphase

We have seen that aqueous micelles are important in all cases in which hydrophobic, lipophilic substances have to be solubilized - and this, as already mentioned, is the basis of the large teclmical importance of micelles in laundry, oil refining, cosmetics, and chemical reactivity at the oil/water interphase. Reverse micelles, as the term implies, are important in the reverse case, when a hydrophilic substance needs to be solubilized in an oily environment. Typical solvents in this case are hydrocarbons, chloroform, or CCI4. 

The experimental methods have been described (1). Films are drawn by passing a hole in a Teflon screen through the oil-water interphase. The oil is a 0.5% solution of purified egg lecithin in a 3 1 vol./vol. heptane-chloroform mixture. By varying the heptane-chloroform ratio the specific density may be varied and thereby the drainage time of the film. The temperature is 23 °C. throughout. 

By subcutaneous administration, the medicine is injected into the subcutaneous coimective tissue. These injections are experienced as more painful than intramuscular ones. Suitable places are the thigh and the belly pleat. The absorption after subcutaneous injection varies in rate and extent depending on the site of injection and on patient specific factors such as the amount of subcutaneous fat and physical activity. When a solution is injected the diffusion of the active substance through the tissue to the blood vessel will be the rate determining step for the absorption. When a suspension is injected the dissolution rate of the active substance may become rate determining for the absorption process, whereas for lipophilic active substances formulated as an oily solution or an oil-in-water emulsion the transport over the oil-water interphase may become rate determining. With suspensions and emulsions the absorption rate can be slowed down to such an extent that it will become rate limiting for the elimination rate of the medicine. In this way slow release injections can be formulated. 

The formation of a microemulsion as a distribution of droplets of either oil or water dispersed in a continuous water or oil phase can be justified in terms of the concentration and interacting energies of the only surfactant at the water-oil or oil-water interphase, under the assumption that the dispersed liquid behaves as a bulk massive phase [17-19]. Therefore the results plotted in Fig. 10 offer rather direct verification of the above working hypothesis. 

Fig. 3.17. A hypothetical model of pancreatic lipase fixation of an oil/water interphase (according to Brockerhoff, 1974)...

Interphase Free-Radical Copolymerization at the Oil-Water Boundary. . 168... 

To elaborate a theory of interphase copolymerization at an oil-water boundary the necessity arises to consider initially the growth of an individual polymer chain near the surface separating the organic and water phases. By the model introduced in paper [74], molecules of only one of the monomers are presumed to be solved inside either of these two phases. A theoretical examination of the formation of macromolecules turns out here to be substantially simpler, since their chemical structure under such an approximation is the same as that of a traditional block copolymer. 

Figure 3. Microemulsion catalysis a, reaction at an interphase b, reaction after transport across the interphase. Key , water soluble ion , water /vws, oil phase molecules , polar organic reactant 9-, surfactant cosurfactant and---------------------------------------------------------, nonpolar organic reactant.

Utilization of mlcroemulslons would appear to be one method by which the polymerization problem might be reduced or even eliminated. In a water-in-oil microemulsion we would expect the ii)-hydroxy- and u-bromoacids to be compartmentalized on a molecular basis l.e. an average of one molecule per drop up to some concentration, then two per drop, etc., and movement between drops inhibited. As a consequence since the base is soluble in water and the acid likely located in the interphase we would expect that the chance of ring closure before dimerization, tri-merization, etc., would be greatly enhanced over that existing in homogeneous media. 

Sol-gel matrices can also provide a chemical surrounding that favors enzymatic reactions. Lipases act on ester bonds and are able to hydrolyze fats and oils into fatty acids and glycerol. These are interphase-active enzymes with lipophilic domains and the catalytic times reaction occurs at the water-lipid interface. Entrapped lipases can be almost 100 times more active when a chemically modified silica matrix is used. The cohydrolysis of Si(OMe)4 and RSi(OMe)3 precursors provides alkyl groups that offer a lipophihc environment that can interact with the active site of Upases and increase their catalytic activity. Such entrapped lipases are now commercially available and offer new possibilities for organic syntheses, food industry, and oil processing. ... 

Another common problem with two phase foods (e.g., mustard) and melted cheese is the formation of a thin water or oil layer next to the walls of the measuring geometry. Because this layer of water or oil has a finite velocity during the experiment (slip), the zero-velocity boundary condition (no slip) at the interphase between the food and the measuring geometry that is used in deriving the equations for shear rate is not satisfied. 

Recently, Steinbach and Sucker (23) reported about the formation of l+-H20-molecule structures that may develop on the hydrophilic groups of surface active compounds upon dilatation of a l-H20-molecu-le structure, by adsorbing 3-water molecules from the subphase at a water-air interface. In the case of the water-oil interphase of the microemulsion, the dispersed droplet consits of an interphasal choro-na that surrounds an inner water core the free water fraction of the latter (bulk-H20)is the subphase that, acting as a reservoir, supplies H2O molecules to the interphase region. Since the formation of hydrated structures takes place at ons ant sur ace tension (23), the above mechanism allows the water-oil interface to expand without affecting the surface pressure necessary to maintain the system s equilibrium. In this way while the area of every polar head of the amphi-phile remains constant, the interphase area stabilized by a single polar head increases up to the amount corresponding to the definite area requirement of the it-H20-molecule structure (23) (3-6). 

Such systems consist of a contineous phase (a), interphase (c) and dispersed phase (b). In liquid scintillators the systems are water-in-oil whereby the dispersed phase represents the aqueous sample. 

The effect of the addition of short chain alcohols on the chromatographic selectivity and peak efficiency was extensively exposed in previous chapters. The addition of such alcohols to a micellar solution forms mixed micelles. This is the first step toward the achievement of microemulsions with ionic surfactants. The oil in water microemulsion (LI structure, see Chapter 2) has a continuous aqueous phase containing oil swollen micelles or microdroplets of oil stabilized by an alcohol-surfactant interphase layer. The medium is transparent and stable, however it 1ms a dynamic structure. Then, it is interesting to see if LI microemulsion mobile phases could be useful in... 

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